In lithographic printing, lithographic ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the hydrophilic surface is moistened with water and lithographic ink is applied, the hydrophilic regions retain the water and repel the lithographic ink and the lithographic ink receptive regions accept the lithographic ink and repel the water. The lithographic ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the lithographic ink can be first transferred to an intermediate blanket that in turn is used to transfer the lithographic ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer (processing solution), revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the precursor is considered as positive-working. Conversely, if the non-exposed regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are lithographic ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel lithographic ink.
“Laser direct imaging” methods (LDI) are used to directly form an offset lithographic printing plate or printing circuit board using digital data from a computer. There have been considerable improvements in this field from use of more efficient lasers, improved imageable layer compositions and components thereof, and improved processing solutions (developers) and procedures.
Various radiation-sensitive compositions are used in negative-working lithographic printing plate precursors as described in numerous publications such as U.S. Pat. No. 7,097,956 (Miyamoto et al.). Many other publications provide details about such negative-working radiation-sensitive compositions comprising necessary imaging chemistry dispersed within suitable polymeric binders. After imaging, the negative-working lithographic printing plate precursors are developed (processed) to remove the non-imaged (non-exposed) regions of the imageable layer.
Negative-working lithographic printing plate precursors that can be imaged and processed on-press during lithographic printing are also known to contain particulate polymeric binders. On-press developable imaged precursors also depend upon properties of the lithographic printing ink/fountain solution emulsion to develop (remove) non-exposed regions during the initial printing impressions. The resulting lithographic printing plates should exhibit good ink receptivity in the imaged (exposed) regions of the printing surface and at the same time exhibit sufficient developability in the non-exposed (non-imaged) regions, especially in the shadows of the printing image.
For on-press developable negative-working lithographic printing plate precursors, the use of polymeric binder in the negative-working imageable layer comprising pendant polyalkylenes oxide segments, as described for example in U.S. Patent Application Publication 2005-0003285 (Hayashi et al.), have been found to be especially useful to provide solvent resistance and higher run length. Such polymeric binders can be present in particulate form. The benefit of using binders in particulate form over using non-particulate binders is described in U.S. Pat. No. 6,899,994 (Huang et al.).
Negative-working lithographic printing plate precursors that can be imaged and processed on-press can also contain various particulate organic or inorganic filler materials in the imageable layers as described in U.S. Pat. No. 7,361,451 (Oshima et al.). As it can be seen from the working examples and comparative examples of this publication, the primary effect alleged to be achieved from addition of the filler materials is faster development speed on press. The filler materials used in the examples include those with particle size as big as 3 μm in Example 7 that is far above the average dry thickness of 1.2 μm, and those with particle size as small as 0.02 μm as in Example 5 is far below the average dry thickness of 0.7 μm. It can also be seen from these examples that the presence of the filler materials has little or no effect on the printing durability. Furthermore, all examples used binders in non-particulate form or microcapsules that encapsulate polymerizable monomers or other components that are essential for free radical polymerization.
In order to reduce the energy requirements for imaging negative-working lithographic printing plate precursors using infrared lasers, processing on-press, and providing adequate printing durability on press, various advanced free radical initiator compositions have become available. For example, a free radical initiator composition comprising iodonium-borate complex is taught in U.S. Pat. No. 7,524,614 (Tao et al.). Such advanced initiator compositions allow high speed imaging due to reduced energy requirement even without an oxygen barrier layer (protective overcoat) that is typically applied over an imageable layer that relies on imagewise hardening via free radical polymerization. For on-press development applications, the presence of an oxygen barrier layer is undesirable as it inhibits printing inks from attaching to the imageable layer in the laser exposed area and increases the number of printing sheets before such sheets become sellable. The primary components in the oxygen barrier layer are typically soluble in fountain solutions used in printing operation. Accumulation of such materials change the composition of fountain solutions and may break the ink-water balance during normal printing operation, leading to printing failures such as image “blinding” (inks failing to attached to the imaged areas) or “scumming” (inks attached to the non-printing surface of the printing plate), or both.
One of the disadvantages of using such advanced initiator compositions such as iodonium-borate compositions is that such compositions tend to crystallize on the surface of the imageable layer, a phenomenon common known as “blooming” in the art. Blooming is particularly severe when particulate primary binders are used, as the initiator composition can only be located between the primary binder particles and have higher local concentration than in situations where the primary binders are in non-particulate form. Blooming occurs more readily when the surface of the imageable layer is touched by various face rollers used in the printing plate precursor manufacturing. Thus, in order to minimize blooming, the average dry thickness of the imageable is typically kept low relative to the roughness of the underlying substrates. However, low average dry thickness often leads to low printing durability. As a result, on-press developable printing plate precursors have been limited to short to medium run length printing applications. Thus, in order to expand the applicability of on-press developable printing plate precursors, there is a need to further improve the printing durability while maintaining other essential properties such as fast imaging speed, fast on-press development, minimal contamination to the fountain solution, and good thermal stability during storage of the printing plate precursor.